1. Field of the Invention
The invention relates to a charging apparatus and a charging method. More particularly, the invention is suitable when it is applied to a charging apparatus and a charging method of a secondary battery of a cellular phone or the like having two or more input terminals for charging.
2. Description of the Related Arts
A primary battery such as a dry battery, and a secondary battery, such as nickel-cadmium battery, nickel-hydrogen battery, or lithium-ion battery, or the like, are used as a power source of a portable electronic apparatus. Even if the secondary battery is dead, the battery can be repetitively used by being charged. However, it is very tiring that each time the battery is dead, the user removes the secondary battery from the portable electronic apparatus and charges it by a charger. Therefore, a cellular phone or the like has a structure such that it can be charged in a state where the secondary battery is held in the main body.
As a method of charging the cellular phone in a state where the secondary battery is held in the main body, for example, there is a method whereby a home-use, commercially available, AC power source is used as an external power source, and the charge is performed by using an AC adaptor or a method whereby a battery of an automobile is used as an external power source and the charge is performed via a cigarette socket, a vehicle-mounted adaptor, or the like. Among recent portable electronic apparatuses, there is an apparatus having a socket into which a terminal of the adaptor for each of those external power sources is directly inserted and the charge is performed. Further, there is an apparatus, such as cellular phone, MD (mini disc) player, or the like, such that merely by putting it onto a holder connected to the adaptor for the external power source, the charge can be performed.
The ordinary cellular phone has two charging terminals, such as a charging terminal for the holder and a charging terminal for the adaptor, so that in a use location, such as a home or the like, importance is attached to the easiness of the charging operation and the charge can be performed via the holder connected to the adaptor for the external power source, and in a mobile location, such as outdoor, room in an automobile, or the like, importance is attached to the easiness of carrying and the charge can be performed only by the adaptor for the external power source. In order to make the apparatus cope with such a plurality of charging methods, by providing a plurality of charging input terminals for the portable electronic apparatus, the user can easily charge it.
A charging apparatus of such a conventional, portable electronic apparatus having a plurality of charging terminals will now be described. FIG. 1 is a diagram showing an example of a conventional charging circuit of a cellular phone. The charging circuit is constructed of: a terminal 101 for an external power adaptor; a terminal 102 for a holder; diodes D101 and D102; a pnp-type transistor Q101; resistors R101 and R102; a control IC 103; and a secondary battery E101.
The terminal 101 for the external power adaptor is an input terminal for directly connecting a connecting terminal of the adaptor for the external power source and inputting a DC power source converted by the external power adaptor. The terminal 101 for the external power adaptor comprises a terminal 101a of a plus (+) electrode and a terminal 101b of a minus (−) electrode. The terminal 102 for the holder is an input terminal for connecting to a terminal of the holder to which the adaptor for the external power source has been connected and inputting a DC power source converted by the adaptor for the external power source via the holder. The terminal 102 for the holder comprises a terminal 102a of a plus electrode and a terminal 102b of a minus electrode. The secondary battery E101 is a battery, such as lithium-ion battery, nickel-hydrogen battery, or the like, of what is called a cellular phone.
The terminal 101a is connected via the diode D101 and the terminal 102a is connected via the diode D102 to an input electrode, that is, an emitter of the pnp-type transistor Q101, and the control IC 103, respectively. An output electrode, that is, a collector of the pnp-type transistor Q101, is connected to a plus side of the secondary battery E101 via the resistor R101. Both ends of the resistor R101 are connected to the control IC 103. Further, a control electrode, that is, a base of the pnp-type transistor Q101, is connected to the control IC 103 via the resistor R102. A minus side of the secondary battery E101 and a part of the control IC 103 are connected to the terminals 101b and 102b. 
In the charging circuit, if a current of an external power voltage is inputted from the terminal 101a, the diode D101 is turned on by a forward bias, so that the current of the power voltage is supplied to the pnp-type transistor Q101 and the control IC 103. When the control IC 103 detects the voltage as mentioned above, the current is supplied to the base of the pnp-type transistor Q101 via the resistor R101 under control of the control IC 103. Thus, the pnp-type transistor Q101 is turned on, a portion between the emitter and the collector of the pnp-type transistor Q101 is made conductive, and the secondary battery E101 is charged via the resistor R101.
If the current of the external power voltage is inputted from the terminal 102a, the diode D102 is turned on by a forward bias, so that the current of the power voltage is supplied to the pnp-type transistor Q101 and the control IC 103. When the control IC 103 detects the voltage as mentioned above, the current is supplied to the base of the pnp-type transistor Q101 via the resistor R101 under the control of the control IC 103. Thus, the pnp-type transistor Q101 is turned on, the portion between the emitter and the collector of the pnp-type transistor Q101 is made conductive, and the secondary battery E101 is charged via the resistor 101.
In the case of charging by the charging circuit, a constant voltage, a constant current, or the like to the secondary battery E101 is measured and the charging circuit is controlled by the control IC 103 so that the charge is performed by a desired voltage or current. For example, if a full charge is detected, the control IC 103 stops the current supply to the base of the pnp-type transistor Q101 and stops the charge.
As mentioned above, in the charging circuit shown in FIG. 1, when the current of the external power voltage is inputted from the terminal 101a, the diode D101 is turned on by the forward bias and the current of the power voltage is supplied to the pnp-type transistor Q101 and the control IC 103. At this time, since a backward bias is applied in the diode D102, the diode D102 is turned off and the current is hardly supplied to the terminal 102a. Similarly, when the current of the external power voltage is inputted from the terminal 102a, the diode D102 is turned on by the forward bias and the current of the power voltage is supplied to the pnp-type transistor Q101 and the control IC 103. At this time, since a backward bias is applied in the diode D101, the diode D101 is turned off and the current is hardly supplied to the terminal 101a. 
That is, a situation such that when the external power source is inputted from one of the input terminals for charging, it is outputted from the other input terminal for charging is prevented by using unilaterality of the diode.
However, the foregoing conventional charging apparatus and charging method have the following problems. FIGS. 2A and 2B are diagrams showing an example for explaining a voltage and a current between a diode and a transistor. FIG. 2A is a diagram regarding the voltage, and FIG. 2B is a diagram regarding the current. As shown in FIG. 2A, when a voltage that is obtained after the output from the diode is assumed to be Vq and a voltage that is inputted to the diode D101 is assumed to be Vd1, it is necessary to set Vd1 to be higher than Vq in order to obtain a predetermined voltage Vq. Similarly, when a voltage that is inputted to the diode D102 is assumed to be Vd2, it is necessary to set Vd2 to be higher than Vq in order to obtain the predetermined voltage Vq. This is because a voltage drop (drop voltage) is caused by the diode.
Therefore, the diode is used so that the power voltage inputted to the charging circuit is not outputted from the other input terminal for charging, and in order to obtain the predetermined voltage, it is necessary that the input voltage to the diode is set to be higher by the drop voltage. There is, consequently, a problem such that the efficiency of the charging circuit deteriorates.
As shown in FIG. 2B, when the power voltage is inputted to both of the diodes D101 and D102, when a current flowing in the diode D101 is assumed to be Id1 and a current flowing in the diode D102 is assumed to be Id2, a merged current Iq is equal to Iq=Id1+Id2 (a base current of the transistor Q101 serving as an operation current is ignored).
Therefore, when the power voltages are simultaneously inputted to the charging circuit from a plurality of charging terminals, since power source currents also flow from a plurality of charging terminals, an overcurrent flows to the secondary battery and the charging circuit and an adverse influence is exerted thereto. There is a problem that there is a possibility of deterioration in safety of the charging apparatus.